.NET Framework 3.5 includes cumulative updates based on previous versions of the platform 2.0 and 3.0. You may need it to install and use relatively old programs.
.NET Framework 3.5 is a comprehensive solution to the problem of programming language compatibility. It contains a compiler and a virtual machine. The former translates the code into a Windows-understandable command language for execution. The virtual machine is the environment in which the programs and games themselves are executed.
The platform not only ensures the compatibility of the programs with Windows, but also checks the security of the code. It consists of the Common Language Runtime (CLR) and the Framework Class Library (FCL). CLR is a virtual machine, the core of the .NET Framework. The FCL is a library which interprets the code. It creates the algorithm to pass to the CLR: a series of commands that the OS understands.